The hormonal regulation of molting and seasonal pelage changes will be studied in the Djungarian hamster (Phodopus sungorus). This species maintans a gray coat during the summer breeding season, and on exposure to short daylengths undergoes gonadal regression followed by a molt and the development of a white winter coat. After approximately 6 months in short days the animals show a "spontaneous' gonadal recrudescence and a molt to the dark summer pelage. Serum levels of several hormones will be measured to determine which hormone levels change in association with the fall and spring molts. Also, by manipulating the levels of various hormones independently, we will explore the hypothesis, based on preliminary data, that changes in the serum concentrations of prolactin and gonadal hormones are important in the regulation of pelage changes. In vitro studies with cultured hair follicles will complement the in vivo experiments. In these studies, possible direct effects of pineal melatonin as well as pituitary and gonadal hormones on hair follicle melanogenesis will be examined. Of particular interest will be an examination of the response of cultured hair follicles to daily melatonin pulses of measured duration.